May 20 2008
May 20 2008
Kathy Scanlon navigates the deep ocean
This maps shows what the state of our knowledge about Sars Seamount was before we mapped it with multibeam sonar.
Our first few passes over Sars Seamount with the Multibeam system produced this incomplete map.
With easy access to online mapping websites like MapQuest and Google Earth we have become accustomed to knowing exactly where things are in our world and what they look like. That’s not the way it is in the deep ocean, however. For example, we knew about the existence of Sars Seamount because an expedition of the research vessel Eltanin had collected a specimen of the coral Desmophyllum dianthus from the seamount in 1962, but navigation in 1962 was most likely done by sextant rather than satellite and had much less accuracy than we have today. The Eltanin’s location for Sars Seamount differed by as much as 17 kilometers from the locations shown on maps and charts of the area. So, before we could try to dredge for fossil corals on Sars Seamount, we had to find it!
We approached the area where we thought Sars should be with our multibeam mapping system on. The multibeam system sends a fan of sonar signals toward the seafloor on each side of the ship and transducers mounted in the hull of the ship listen for the echoes. The signal is then processed to produce a swath of bathymetry that under ideal conditions can be four times as wide as the water is deep. (For example, if the water is 3000m deep we could map a path 12000m wide across the seafloor. In the real world, we get much less than that because the seas are never flat, the ship rolls, air bubbles get under the hull, wind and currents make the ship list, etc.) Our first trackline missed the top of the seamount by a couple of kilometers, but we mapped enough of it to know where to go to find the top. Just as the locations of Sars varied among maps, so did the water depth. The existing maps showed the top of the seamount to be anywhere from 1550m to 217m deep! Now, after a day of mapping with multibeam sonar, we know that Sars Seamount rises 3000 meters above the surrounding seafloor to a depth of 500 meters below the sea surface. The sides are full of ridges and gullies and numerous knolls contribute to a complex morphology. It’s a far cry from the smooth contours on the previously existing maps!
IT technician Mike Coons checking the multibeam and Knudsen.
John Swartz, Kathy Scanlon and Kate Hendry working on the ships map table.
Tina van de Flierdt, John Swartz and Kathy Scanlon in their matching 'Extreme Cole weather' gear issued by the US Antarctic Program.
John Swartz, Kathy Scanlon and Kate Hendry sorting through corals, shells and bryozoans from the first dredge.